Australian Ford Fairmonts 1996

Good Day, I and a friend have 1996 Ford Fairmonts imported from Australia. We have both had an intermittant problem with the vehicles misfiring from time to time. We have both tried all the normal fixes. ie. plugs, plug leads etc. and can not find the problem. Unfortunately Ford South Africa do not import these anymore and carry almost zero spares. And finding a mechanic that knows these engines is nigh impossible ! 4.0L straight six.
Is there anybody that has the solution to this problem or can suggest any possible fixes ?
I am mechanically minded albeit limited but my friend is a retired chief engineer with plenty of experience. Looking for suggestions at least, thank you. Dave Melly